It is well known that carbon fibers having excellent properties suitable for commercial use can be produced from mesophase pitch. Mesophase pitch derived carbon fibers are lightweight, strong, stiff, electrically conductive, and both chemically and thermally inert. The mesophase derived carbon fibers perform well as reinforcements in composites and have found use in aerospace applications and quality sporting equipment.
Carbon fibers have been made commercially from three types of precursor materials, rayon, polyacrylonitrile, and pitch. The use of pitch as a precursor material is attractive economically.
Low cost carbon fibers produced from isotropic pitch exhibit little molecular orientation and relatively poor mechanical properties. In contrast carbon fibers produced from mesophase pitch exhibit high preferred molecular orientation and relatively excellent mechanical properties.
As used herein the term "pitch" generally refers to a carbonaceous residue consisting of a complex mixture of primarily aromatic organic compounds which are solid at room temperature and exhibit a relatively broad melting or softening temperature range. When cooled from the melt the pitch is solidified without crystallization.
The term "mesophase" is to be understood as used in the prior art and is synonymous with liquid crystal, that is a state of matter which is intermediate between crystalline solid and an isotropic liquid. Ordinarily material in the mesophase state exhibits both anisotropic and liquid properties.
As used herein the term "mesophase pitch" is a pitch containing more than about 40 percent by weight mesophase and is capable of forming a continuous anisotropic phase when dispersed by agitation in accordance with the prior art.
A number of methods for preparing mesophase pitch have been disclosed in the prior art. Generally they involve (a) isolation of an aromatic resin containing material, and (b) heat soaking in combination with gas sparging or solvent fluxing to produce a mesophase pitch.
Ashland A240 pitch, a commercial petroleum pitch, is commonly used as a mesophase pitch precursor. It has a high resin content and produces mesophase pitch yields as high as 50 percent. Ashland A240 pitch contains a moderate amount of solids (typically 150 ppm ash) which diminishes the value of the carbon fibers it produces.
There are a wide variety of feedstocks which can be used for the preparation of petroleum mesophase pitch. Petroleum decant oils and petroleum thermal tars are suitable feeds but they generally produce low mesophase pitch yields (approximately 10 percent). Also pitches from these feeds usually contain large amounts of undesirable solids.
Coal tar pitches are usually very high in carbonaceous solids which must be removed before they are acceptable mesophase pitch precursors.
Aromatic distillates have been thermally treated to make resin containing pitches. Short, hot, high pressure thermal treating makes suitable material not unlike petroleum decant oil or thermal tar. Carbonaceous material usually contaminates the products when conditions are severe enough to make more than a low yield of resin material.
All of the above feed materials contain either solids such as ash, or catalyst fines, or a carbonaceous material such as coke, all of which have adverse effects on the properties of carbonaceous products produced from these feeds. This limits their usefulness as impregnation or binder pitches and adversely affects the properties of carbon fibers made from mesophase pitch prepared from these materials.
According to the present invention, a mesophase pitch substantially free from carbonaceous insolubles and from ash and ash catalyst fines and other solids is obtained by distilling an aromatic containing feedstock to obtain a distillate which is free from mesophase and mesophase forming resins. The distillate is heat soaked without gas sparging at elevated temperatures for a sufficient period of time to obtain a heat soaked distillate which is free from mesophase but contains at least 5 percent mesophase forming resins. The heat soaked distillate is further heated at elevated temperatures with inert gas sparging to convert it to mesophase pitch suitable for the manufacture of carbon fibers. Alternatively the heat soaked distillate may be used as a binder or impregnation pitch.